It is frequently desirable to reinforce rubber articles, for example, tires, conveyor belts, power transmission belts, timing belts, hoses and the like products, by incorporating therein steel reinforcing elements. Pneumatic vehicle tires are often reinforced with cords prepared from brass-coated steel filaments. Such tire cords are frequently composed of high carbon steel or high carbon steel coated with a thin layer of brass. Such a tire cord can be a monofilament, but normally is prepared from several filaments which are stranded or bunched together. In some instances, depending upon the type of tire being reinforced, the strands of filaments are further cabled to form the tire cord.
It is important for the steel alloy utilized in filaments for reinforcing elements to exhibit high strength and ductility as well as high fatigue resistance. Unfortunately, many alloys which possess this demanding combination of requisite properties cannot be processed in a practical commercial operation. The alloys which have proved to be commercially important have typically required a patenting procedure wherein they are subjected to an isothermal transformation from austenite to pearlite. U.S. Pat. No. 5,167,727 describes such a process wherein steel filaments are manufactured utilizing a patenting step wherein the transformation from austenite to pearlite is carried out under isothermal conditions at a temperature which is within the range of about 540.degree. C. to about 620.degree. C. Such isothermal transformations are normally carried out in a fluidized bed or in a molten lead medium to maintain a constant temperature for the duration of the transformation. However, the utilization of such an isothermal transformation step requires special equipment and adds to the cost of the patenting procedure.
A fine lamellar spacing between carbide and ferrite platelets in the patented steel wire is required to develop high tensile strengths while maintaining the good ductility required for drawing the wire. To achieve this goal, small quantities of various alloying metals are sometimes added to the steel in order to improve the mechanical properties which can be attained by using isothermal patenting techniques.
An alternative to isothermal patenting is continuous cooling or "air" patenting. In this process, high carbon steel wire is allowed to cool in air or other gas, such as cracked ammonia, which can be either still or forced in order to control the rate of cooling. This process typically produces a microstructure which has a lamellar structure which is somewhat coarser than that achieved with isothermal patenting. As a result, the tensile strength of the wire is significantly lower than that achieved by isothermal patenting and filaments drawing from the wire have lower tensile strengths. An additional drawback to the use of continuous cooling in patenting procedures is that as the diameter of the wire increases, the rate at which the wire cools is reduced and the microstructure becomes even coarser. As a result, it is more difficult to produce wires of a larger diameter with acceptable properties.